1. Field of the Invention
This invention generally relates to a linear motion rolling contact guide unit, and, in particular, to a retainer for use in a finite stroke type linear motion rolling contact guide unit for retaining its rolling members in position.
2. Description of the Prior Art
A linear motion rolling contact guide unit is well known in the art, and, it generally includes a rail extending straight over a desired length, a slider slidably mounted on the rail and a plurality of rolling members interposed between the rail and the slider. Such a linear motion rolling contact guide unit can be either of the infinite stroke type or of the finite stroke type. In the former case, an endless circulating path, in which the rolling members are provided, is provided between the rail and the slider so that theoretically an infinite relative motion can be provided between the rail and the slider as long as the rail extends. On the other hand, in the latter case, only a straight guide channel is provided between the rail and the slider in which the rolling members move rolling back and forth so that only a finite stroke can be provided for a relative movement between the rail and the slider.
A finite stroke type linear motion rolling contact guide unit for which the present invention can be advantageously applied is illustrated in FIGS. 6 and 7. As shown, the finite stroke type linear motion rolling contact guide unit includes a bed or rail 28 extending straight over a desired length and having a generally rectangular cross section, a table or slider 29 slidably mounted on the rail 28, and a plurality of rolling members or balls 31 in the illustrated embodiment interposed between the rail 28 and the slider 29.
The rail 28 is formed with a pair of inner guide grooves 35 located one at each side surface thereof and extending in parallel with the longitudinal axis of the rail 28. The slider 29 has a horizontal section extending horizontally over the rail 28 and a pair of vertical sections each depending from a corresponding side of the horizontal section and spaced apart from the associated side surface of the rail 28. An inner guide groove 34 is formed at an inner side surface of each of the vertical sections of the slider 29 in an opposed relationship with the associated inner guide groove 35 to thereby define a straight guide channel between the rail 28 and the slider 29.
A plurality of balls 31 as rolling members are provided in each of the guide channels such that they are partly received in each of the oppositely arranged inner and outer guide grooves 35 and 34. Also provided in each of the guide channels is a retainer 30 for retaining the balls 31 spaced apart from each other and in position. The retainer 30 includes an elongated plate formed with a plurality of pockets 32 spaced apart from each other at an equal interval each receiving therein one of the balls 31 rotatably. The balls 31 may be maintained in position in the guide channel by the retainer 30 even if the slider 29 moves relative to the rail 28. However, since each of the pockets 32 is substantially identical in shape and size, and the pockets 32 are circular in shape and approximately equal to the diameter of the balls 31 with a slight gap therebetween so as to allow the ball 31 to rotate therein.
However, such a structure has been found to be disadvantageous in some cases, in particular when used under a preloaded condition, because of an increased sliding resistance between the rail 28 and the slider 29. When preloaded, the internal gap between the balls 31 and each of the rail 28 and the slider 29 is set to be negative in value as shown in FIGS. 6 and 7, and a linear motion guide unit having such a preloaded condition can provide an enhanced accuracy and smooth operation. Several causes are possible in increasing the sliding resistance, such as a slight error in the degree of parallelism between the inner and outer guide grooves 35 and 34, variations in the lubricating condition along each of the guide channels; however, no truly viable solution to keep the sliding resistance lower has been proposed prior to the present invention. It is the present inventor that has pinpointed the major reason for an increase of the sliding resistance as a result of an extensive study regarding the causes of an increase of the sliding resistance between the rail and the slider. That is, according to the study made by the present inventor, it has been found that an interaction between the rolling members, typically balls or rollers, and the retainer plays an important role in the increase of the sliding resistance between the rail and the slider.
Therefore, there has been a need to provide an improved retainer which can allow to keep the interaction with the rolling members as small as possible to thereby keep the sliding resistance of a linear motion guide unit as low as possible at all times. In addition, after using a linear motion guide unit for some time, there may occur a case in which the distance between the center line or longitudinal axis of the rail 28 or slider 29 and the center line of the retainer 30 vary or fluctuate, which is disadvantageous because of a degradation of the accuracy in linear motion. Thus, there also has been a need to solve such a problem.